Electrical power supplies commonly use diode rectifier circuits to convert from alternating current (AC) to direct current (DC). A diode rectifier conducts current only when the input voltage of the rectifier exceeds the output voltage of the rectifier, so a sinusoidal input voltage results in intermittent non-sinusoidal current flow. The intermittent current flow has a primary frequency component equal to the AC input frequency and substantial energy at integer multiples of the AC input frequency (harmonics). Input current harmonics can cause transient current flow in the AC mains, which can increase the power required from the AC mains and can cause heating of the distribution system. In addition, input current harmonics create electrical noise that can interfere with other systems connected to the AC mains. Increased power, heating, and electrical noise are especially important considerations for Uninterruptable Power Supply (UPS) systems used to provide AC power in large computer server systems.
The power factor of a power supply is the ratio of the real power delivered to a load divided by the apparent input power, where the apparent input power is the Root-Mean-Square (RMS) input voltage times RMS input current. In general, input current harmonics cause the RMS value of the input current to be substantially higher than the current delivered to the load. Many power supplies include power factor correction to reduce input current harmonics. Some jurisdictions legally require power factor correction for supplies with output power over a specified limit, which includes most power supplies for computer systems.
FIG. 1A illustrates an example of part of a power supply 100 (simplified to facilitate illustration and discussion) with conventional power factor correction. An AC input voltage Vi is rectified by a full-wave rectifier 102. An inductor 104 provides energy storage to enable a continuous input current. A power factor correction (PFC) module 106 controls an electronic switch 108 using pulse-width-modulation (PWM) to control the DC output voltage VB and to generate a continuous sinusoidal input current in phase with the input voltage Vi. The circuit illustrated in FIG. 1A may be a front end to a DC-DC converter. Alternatively, there may be multiple inductors and switches driving multiple DC outputs, which in turn may connect to multiple DC-DC converters.
FIG. 1B illustrates an example of additional detail for the PFC module 106 in FIG. 1A. The output bus voltage VB is subtracted from a reference voltage VREF at a summing node 110. The resulting voltage error signal is input to a voltage loop controller 112, which regulates the bus voltage VB to be equal to VREF. An RMS calculator 114 computes the inverse of the square of the RMS value of the input voltage Vi. The output of controller 112 is multiplied by 1/Vi2RMS by a multiplier 116. That result is multiplied by the input voltage Vi by a multiplier 118 with a gain of K, the value of which depends on system parameters. That result is used as a sinusoidal reference signal iREF for a current control loop. Sensed current ISEN is subtracted from the reference current signal iREF at a summing node 120, and resulting current error signal is processed by a current loop controller 122, and the result is used to generate the PWM output that controls the electronic switch 108.
For some power supplies, such as power supplies used for computer servers, the operating conditions may vary widely, with input voltages ranging from 90V to 264V, and output loads varying from zero to full load. Under such varying operating conditions, the PFC power stage characteristics can change significantly, which results in a corresponding significant change in gain, bandwidth, and stability margins (phase margin and gain margin) of the current control loop, making it difficult to achieve a good power factor and low input current total harmonic distortion (THD) under all operating conditions, especially for light loads and high input voltages.